1. Field of the Invention
The present invention relates to a thermally driven (or thermally operated) liquid pressure generating apparatus which is suitable as a liquid pressure source for a fluid actuator which is mounted on a vehicle such as a motor vehicle.
2. Description of the Related Art
In case a fluid actuator such as a hydraulic cylinder for adjusting a vehicle height, a hydraulic cylinder for steering rear wheels in a four-wheel-steering vehicle, or the like, is mounted on a vehicle, there has conventionally been used, as a liquid pressure source for the fluid actuator, a pump which is driven by an electric motor or an engine. This arrangement, however, has a disadvantage in that the energy consumption increases and the specific fuel consumption becomes poor.
Therefore, as a liquid pressure source which serves as an alternative for the pump, it is desired to use a thermally driven liquid pressure source which generates the liquid pressure by utilizing a waste heat of the driving source which is mounted on the vehicle.
As this kind of liquid pressure generating apparatus, the following is considered. Namely, a medium which changes the phase between a gaseous phase and a liquid phase is filled in a vessel containing therein a liquid chamber which is capable of changing its own volume. At the bottom of the vessel, there is provided a heat exchange portion to which is inputted an external heat, i.e., the waste heat from the driving source. The liquid-phase medium which stays at the bottom of the vessel is heated to change its phase to a gaseous phase. The liquid chamber is thus compressed by the vapor pressure of the medium to thereby generate the liquid pressure.
The above-described thermally driven liquid pressure generating apparatus has the following disadvantages. Namely, since the internal volume of the vessel in other than the liquid chamber increases as a result of compression of the liquid chamber, the vapor of the medium adiabatically expands to thereby lower in temperature, with the result that the vapor pressure lowers or decreases and further that the medium condenses. It thus takes time to increase or boost the liquid pressure. Further, even if the heat input to the heat exchange portion has been stopped, the vapor pressure does not lower or decrease much until the entire vessel has cooled down, with the result that the decrease in the liquid pressure also takes time.
In view of the above-described disadvantages, the present invention has an object of providing a thermally driven liquid pressure generating apparatus in which the liquid pressure can be increased or decreased with a good response.